Thermal printhead operation

ABSTRACT

A method of operating a thermal printhead includes the steps of periodically determining a current value proportional to the resistance of printhead elements; providing a warning signal when the current value of any element exceeds a predetermined multiplier of the nominal value; then modifying the operation of the printhead so that such element is energised only for the printing of a barcode within the printed area. Also there is apparatus for carrying out such a method. The method and apparatus may be applied to a weighing apparatus.

[0001] Thermal printers arc used extensively in situations such as theretail trade, for printing product labels, for example.

[0002] The labels may have printed on them such things as textualinformation, logos and product bar codes as a series of dots.

[0003] A thermal printer is a type of printer in which the image isproduced by localized heating of paper that has a very thinthermosensitive coating containing two separate and colourlesscomponents, a colour former and a dyestuff. The heating is by elementsor “dots” typically arranged in a row in a printhead. When heated thecolour former melts and combines with the previously colourless dyestuffto make a visible mark.

[0004] The aim of the present invention is to provide automaticindication of imminent printhead dot failure on a thermal printhead. Inaddition, the period in which the printhead is able to produce“scannable” bar codes will be extended by selective use of printheaddots once wear has been indentified.

[0005] By way of example only, weighing machines often employ thermalprintheads in order to print label or receipt information. The textualinformation is usually important, however, should some dots fail toprint, under normal circumstances that information will still belegible, However, scanning of barcodes, when barcodes are printed inline with the printhead ‘dots’ generally will not tolerate any dotfailures, especially when the scanners deployed to scan the barcodes arcof a hand-held technology.

[0006] It is frequently difficult and inconvenient to change a printheador to take a weighing machine out of service immediately. Also asituation which would require recalibration using the actual resistancesof the elements of a printhead at installation of a replacementprinthead raises problems due to users changing faulty print heads andsuch calibration has been found to be unnecessary.

[0007] Consequently it is preferable not to rely on calibrated valuesand to avoid the necessity of changing a print head for as long aspossible.

[0008] The object of the present invention is to attempt to provideautomatic identification of imminent element failure and to extend theability to print the most critical trading information (i.e. thebarcodes). This will reduce significantly the number of mis-scans, whichwill increase the through-put of retail checkouts and reduce publicfrustration when performing barcode self-scans.

[0009] According to the present invention there is provided:—

[0010] a method of operating a thermal printhead comprising the stepsof:—

[0011] (a) periodically determining a current value proportional to theresistance of each element within the thermal printhead;

[0012] (b) providing a warning signal when the current valueproportional to the resistance of one or more elements exceeds apredetermined multiplier of the nominal value of the resistance of thatelement:

[0013] (c) In response to said warning signal modifying the operation ofthe printhead whereby an element or elements whose current valueproportional to the resistance thereof exceeds a predeterminedmultiplier of the nominal value of the resistance of that element isenergised only for the printing of a barcode.

[0014] There is further provided apparatus for operating a thermalprinthead, comprising switching means for entering a self-test routineduring a period of inactivity of the printhead and sequentiallydetermining current values proportional to the current resistance ofeach element, a comparator for comparing a stored nominal value for eachelement and the current value for each element and control meansproviding a warning signal when the current value for that elementexceeds a predetermined multiplier of the nominal value for that elementand for controlling the energising of an element or elements to occuronly when printing barcodes when the current value for an element hasexceeded a predetermined multiplier of the nominal value for thatelement.

[0015] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:—

[0016]FIG. 1 shows a block schematic of an apparatus in accordance withthe present invention;

[0017]FIG. 2 shows a typical graph of the element resistances of a newprinthead; and

[0018]FIG. 3 shows a typical graph of the element resistances in aprinthead where for a number the elements the resistance has increasedto the point where failure is likely to be imminent.

[0019]FIG. 4 shows a typical lable when printhead dot wear exists andprinting is occuring in the barcode area and not in the other areas.

[0020] A printhead provides a series of dots forming a dot line. Asshown in FIG. 1, using a fixed resistor as a reference, a potentialdivider is formed by this and the individual dots. The potentialdifference measured by the A-to-D circuitry is approximatelyproportional to the dot resistance.

[0021] During periods of inactivity, the weighing equipment willautomatically enter a user transparent_self-test routine. Thc resistanceof each element will be measured in turn and compared with the nominalprinthead value. Any dot which is outside its resistance specificationwill be deemed as “imminently about to fail”. This will result in awarning signal being generated and provided to any or all of thefollowing:

[0022] (a) the user (via weighing equipment display);

[0023] (b) a rear-of-store computer

[0024] (c) a head office computer.

[0025] During normal operation of the equipment, at pre-defined regularperiods, the printhead element resistances will be measured and comparedwith the nominal printhead values by the comparator. The example shownin FIG. 3 demonstrates that some dots have now changed resistance by asignificant amount. If this variation is now approaching themanufacturer's defined limit then the equipment will indicate that theprinthead needs to be replaced. Manufacturers typically quote 10% to 15%as being the level at which there is a significant risk of failure.

[0026] The comparator will then provide a signal to a fault indicator,which may be located at the printer, or at a rear-of-store orhead-office computer, for example.

1. A method of operating a thermal printhead comprising the steps of (a)periodically determining a current value proportional to the resistanceof each element within the thermal printhead; (b) providing a warningsignal when the current value proportional to the resistance of one ormore elements exceeds a predetermined multiplier of the nominal value ofthe resistance of that element; (c) in response to said warning signalmodifying the operation of the printhead whereby an element or elementswhose current value proportional to the resistance thereof exceeds apredetermined multiplier of the nominal value of the resistance of thatelement is energised only for the printing of a barcode.
 2. A method ofoperating a thermal printhead as claimed in claim 1, wherein the currentvalue proportional to the resistance of each element is determinedduring periods of non-use of the thermal printhead.
 3. A method ofoperating a thermal printhead as claimed in claim 1 or 2, wherein thecurrent values are determined by sequentially applying a voltage acrossa fixed resistor in series with each heating element to form a potentialdivider and measuring the potential difference across the heatingelement.
 4. A method of operating a thermal printhead as claimed inclaim 3 wherein the voltage is applied sequentially to each element bysending a pattern to a shift register connected to the printhead.
 5. Amethod of operating a thermal printhead as claimed in any precedingclaim where the predetermined multiplier is 110% or 115%.
 6. A method ofoperating a thermal printhead as claimed in claim 1 and substantially ashereinbefore described.
 7. Apparatus for operating a thermal printhead,comprising switching means for entering a self-test routine during aperiod of inactivity of the printhead and measuring means forsequentially determining initial and current values proportional to thecurrent resistance of each element, a comparator for comparing a storednominal value for each element and the current value for each elementand control means providing a warning signal when the current value forthat element exceeds a predetermined multiplier of the nominal value forthat element and for controlling the energising of an element orelements to occur only when printing barcodes when the current value foran element has exceeded a predetermined multiplier of the nominal valuefor that element.
 8. Apparatus as claimed in claim 7, wherein means forsequentially determining the initial and current values comprises avoltage source for applying a voltage across a fixed resistorsequentially connected in series with each heating element to form apotential divider and measuring means for measuring the potentialdifference across each heating element.
 9. Apparatus as claimed in claim7 and substantially as hereinbefore described, with reference to and asillustrated in the accompanying drawings. 10 A weighing apparatusincluding a thermal printhead operated in accordance with the method ofany one of claims 1 to
 6. 11. A weighing apparatus including apparatusas claimed in any one of claims 7 to 9.